In a communication system, such as a wireless communication system, devices communicate with one another while managing various parameters associated with a communication link. For example, a wireless station and user equipment (UE) may communicate with one another while managing various parameters, such as power control, that are associated with a communication link. With respect to TDD communication systems, the same frequency band may be used in both uplink and downlink such that channel reciprocity exists. In this regard, the requirement of providing continuous feedback of channel estimates may be unnecessary.
Long Term Evolution (LTE) is one of many communication platforms that support TDD. In LTE, the physical uplink control channel (PUCCH) is part of band edges in a frequency spectrum. For example, in a 10 MHz frequency spectrum, only the two outer resource blocks (e.g., 180 kHz frequency bands) are allocated to the PUCCH. One PUCCH message (e.g., ACK/NACK or channel quality indicator (CQI)) may be sent in one slot on one of the resource blocks and then a frequency hop may be made to the other frequency band to the second slot.
With respect to the PUCCH, power control consists of a closed loop around an open loop point of operation according to the following expression:PPUCCH(i)=min{PMAX,PO—PUCCHPL+ΔF—PUCCH(F)+g(i)[dBm]  Eq. 1,where PPUCCH is the total power, PMAX is the maximum allowed power that depends on the UE power class, PO—PUCCH is a parameter composed of the sum of a 5-bit cell specific parameter PO—NOMINAL—PUCCH provided by higher layers with 1 db resolution in the range of [−127, −96] dBm and a UE specific component PO—UE—PUCCH configured by Radio Resource Control (RRC) in the range of [−8, 7] dB with 1 dB resolution, PL is the downlink path loss estimate calculated in the UE, ΔF—PUCCH(F) corresponds to table entries for each PUCCH transport format (TF) given by the RRC (i.e., an offset for the modulation and coding scheme employed), and g(i) corresponds the current PUCCH power control adjustment. A more detailed description may be found in 3GPP “E-UTRA Physical layer procedures,” TS 36.213 V8.1.0.
The path loss (PL) in Equation 1 is based on the measured path gain of downlink reference symbols. This measurement is typically done over the entire downlink frequency spectrum and is time-filtered, resulting in a slow-fading, frequency averaged gain of which the power control is based.
In 3GPP RAN1 contribution R1-080337, entitled “Fast open loop power control for PUCCH in TDD mode,” Nokia Siemens Networks & Nokia, it is suggested to utilize the channel reciprocity in TDD-mode where the same frequency band is used in the downlink and the uplink. The open loop shall then be faster to follow multipath fading. It is also suggested that the open loop only be based on measurements on the PUCCH frequencies.
The Physical Uplink Shared Channel (PUSCH) in LTE is power controlled in a similar way as PUCCH, as described in 3GPP “E-UTRA Physical layer procedures,” TS 36.213 V8.1.0, with the same path loss based open loop, according to the following expression:PPUSCH(i)=min{PMAX,10 log10(MPUSCH(i))+PO—PUSCH(j)+αPL+ΔTF(i)+f(i)[dBm]  Eq. 2,where PPUCCH is the total power, PMAX is the maximum allowed power that depends on the UE power class, MPUSCH(i) is the size of the PUSCH resource assignment expressed in number of resource blocks valid for subframe i, PO—PUSCH(j) is a parameter composed of the sum of a 8-bit cell specific nominal component PO—NOMINAL—PUSCH(J) signaled from higher layers for j=−0 and 1 in the range of [−126, 24] dBm with 1 dB resolution and a 4-bit UE specific component PO—UE—PUSCH(J) configured by RRC for j=0 and 1 in the range of [−8, 7] dB with 1 dB resolution, and where the pathloss (PL) is the same wideband downlink pilot measure as for PUCCH. The PUSCH may be transmitted on almost the whole band except the band edges where PUCCH is allocated. However, a UE will often be scheduled on only a fraction of the total band allocated to the PUSCH.
Slow fading gain is only an average value calculated over many frequencies. Therefore, when basing the power output on slow fading gain, this can lead to a coarse power control, as well as slow changes to power output. Additionally, in a closed loop scheme (e.g., UE transmits and a base station measures signal-to-noise and transmits back power commands), there exists a delay (e.g., several milliseconds), which in many cases makes it impossible to follow fast fading. In this regard, if the fast open loop power control is based on wideband power, the fast fading will, for most channels, not be captured. FIG. 1 is a diagram illustrating TDD open loop ACK/NACK error rates. As illustrated, if the fast fading can be capture there is a gain (a lower ACK/NACK error rate) compared to a reference case in an open loop power control scheme based on downlink wideband path loss measurements. In an open loop power control scheme, the UE may, for example, perform measurements on the downlink, determine the fading environment, and manage its power output. For example, the UE may manage its output power so that it reaches a certain signal-to-noise ratio. These simulations results provide results for both slow fading and fast fading.
In the case of power control in a LTE communication system, even if the open loop power control is set based on PUCCH channel bands, the difference between fast fading loss on the two PUCCH resource blocks can be large (e.g., 10 dB or more in case of 10 MHz bandwidths). Thus, one measure for both slots may not be desirable. In this regard, to set a common power for both slots that performs well for both ACK/NACK and channel quality indicators (CQIs) can be difficult.